Abstract: Deltaic Sandstone Reservoirs--Exploration Models and Recognition Criteria

Ram S. Saxena

Within the fluviodeltaic complex, the lower deltaic region forms an ideal zone for hydrocarbon generation and entrapment. In the Gulf Coast subsurface, major reserves of hydrocarbons are present in the deposits of this zone. Two factors make the zone prolific: its location directly updip of extensive, organic-rich, prodelta and marine shales (excellent source rocks), and the complex lithologic variability and abrupt lithofacies changes which provide multiple opportunities for favorable sand-shale juxtapositions to form traps. Five genetic types of sandstone bodies commonly form reservoirs within the lower deltaic zone. These are point bars, crevasse splays, distributary-mouth bars, beach barriers, and the reworked deltaic sands. Recognition criteria, idealized vertical se uences, and predictive depositional models have been developed for the subsurface mapping of these sand bodies by comparing the subsurface data with the data and concepts developed from the modern Mississippi delta and the Carboniferous deltaic sequences of the Appalachian Plateau.

Grain-size characteristics and temporal variations of depositional environments provide major bases for the recognition of the sand bodies in the subsurface. These criteria are discernible on electric logs. SP shapes of discrete sand bodies which crudely reflect grain-size variations provide some clues for the recognition of sand-body genesis. More significant insight for predicting stratigraphy and reconstruction of paleogeography is gained, however, by observing, on electric logs, vertical superposition of environmental units which reflect temporal variations of depositional regimes.

Major sedimentary events in deltaic deposits follow one another in a predictable manner with the preceding event in many cases influencing the latter. By understanding the sequential superposition of depositional events in electric logs, processes such as progradations of shorelines, filling of bays with crevasse splays, shifting of major delta sites into bays of older cycles, and finally abandonment and reworking of older delta lobes, so characteristics of modern and ancient deltas easily can be inferred.

A lithogenetic model showing the sequential arrangement of environmental components of the deltaic plain is a valuable predictive tool. Various components of this model can be recognized readily in isolated electric logs and, with minimum control, regional paleogeography and sites of optimum-sand accumulation can be predicted.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC